1. Field of the Invention
The invention relates to a wire electric discharge machine for processing a workpiece (an object to be processed) by repetitively inducing a pulse-shaped discharge between a wire electrode and the workpiece.
2. Description of Related Art
In the conventional wire electric discharge machine, a pulse voltage is applied between a wire electrode and a workpiece by a power source to repetitively induce a pulse-shaped discharge between the wire electrode and the workpiece, whereby the processing of the workpiece is performed.
However, the above type of wire electric discharge machine has the problem that the wire electrode is broken when there is a frequent occurrence of abnormal discharge, having an extremely short no-load time, in which the discharge occurs between the wire electrode and the workpiece simultaneously with the application of a pulse voltage by the processing power source.
In order to solve the above problem, a method, as disclosed in U.S. Pat. No. 4,695,696, which corresponds to Japanese Patent Publication No. 2-44648, has been proposed. In this method, each discharge pulse generated between the wire electrode and the workpiece is discriminated as an abnormal discharge pulse or a normal discharge pulse on the basis of the length of the no-load time and, when an abnormal discharge pulse having short no-load time occurs, the pulse height of discharge current is reduced or a rest time for a pulse voltage supply operation is made longer to thereby reduce the discharge energy supplied between the wire electrode and the workpiece by the processing power source. In that manner, breakage of the wire electrode is avoided.
However, even when the normal discharge is induced, a so-called abnormal discharge pulse having short no-load time is very frequently actually generated.
FIG. 4 shows a typical example of a voltage waveform between the wire electrode and the workpiece to be processed.
In a graph of FIG. 4, the ordinate represents an inter-electrode voltage while the abscissa represents a time. The graph shows time-variation of the potential of the wire electrode with respect to the workpiece when the workpiece is connected to the earth such that it serves as a positive electrode side. As is apparent from FIG. 4, a number of so-called abnormal discharge pulses having short no-load time, as disclosed in U.S. Pat. No. 4,695,696, occur even when the normal discharge is induced. According to an experiment of the inventors of this application, it has been experimentally proven that a generally-defined abnormal discharge pulse is contained in an amount of about 15 to 40% of all discharge pulses even during the normal discharge operation. Therefore, if each discharge pulse is discriminated as an abnormal discharge pulse or a normal discharge pulse on the basis of only the no-load time and the discharge energy supplied between the wire electrode and the workpiece is reduced for every discrimination of the abnormal discharge pulse, it is obvious to conclude that processing speed is also reduced.
In addition, it is unreasonable, in principle, that the abnormal discharge pulses be defined on the basis of only the no-load time. That is, the judgment of abnormality or normality is originally dependent on various conditions, such as a sectional area of the wire electrode, material of the wire electrode, wire tension, discharge energy per shot, and a discharge period. Thus it is impossible to set a fixed threshold level for the no-load time. If the judgment (discrimination) of abnormality or normality is daringly made on the basis of the no-load time irrespective of the above fact, there would occur a case where the processing speed is reduced or a function of preventing the breaking of wires does not normally work when the wire electrode is changed to one having a different structure, or a processing condition such as a discharge period for a target (desired) inter-electrode voltage between the wire electrode and the workpiece to be processed is altered.